You are booked for flying CX between RUH and NRT.
You have the option of: 1) refund without fee. 2) booking a CX flight at a later date and 3) rerouting between RUH and NRT on another airline.
Nowhere does it say that you can change your point of origin. And RUH is open, although few flights.

I guess that you are probably still in UK and had planned to fly to RUH in April. But CX ticket is only concerned with RUH to Tokyo.
Your best bet is to get a refund and make some other arrangements to get to Tokyo.